To realize the recent trend of a high density and a high miniaturization of semiconductor devices, a plasma processing device is used for performing a film forming process, an etching process, an ashing process and the like in a manufacturing process of the semiconductor devices. Especially, in a microwave plasma processing device which generates a plasma by using a microwave, the plasma can be stably generated under a condition of a pressure ranging from a low pressure of about 1˜300 Pa (high vacuum) to an even comparatively high pressure. For the reason, a microwave plasma processing device using a microwave of, e.g., 2.45 GHz has attracted much attention.
Hereinafter, such a conventional plasma processing device will be described. As shown in FIG. 22, the plasma processing device includes a chamber 101 for accommodating therein a substrate 111 and performing a predetermined process thereon; a high frequency power supply 105 for generating a microwave; and an antenna unit 103 for radiating the microwave into the chamber 101.
The antenna unit 103 is constructed to have a slot plate 103c, a wave delay plate 103b and an antenna cover 103a. Provided in the slot plate 103c is a plurality of slots (openings) through which the microwave is radiated into the chamber 101. The microwave generated by the high frequency power supply 105 is propagated through a waveguide 106 to the antenna unit 103.
Provided at an upper portion of the chamber 101 is a top plate 104 constituting a part of a partition wall of the chamber 101. The top plate 104 is formed of, e.g., a dielectric material such as quartz. Between the top plate 104 and the partition wall of the chamber 101, a sealing member 114 such as, e.g., an O-ring is installed. The antenna unit 103 is disposed above the top plate 104.
In the chamber 101, a susceptor 107 for holding the substrate 111 accommodated in the chamber 101 is installed. Further, a vacuum pump 109 for evacuating the chamber 101 is connected to the chamber 101.
In the above-mentioned plasma processing device, the chamber 101 is evacuated by the vacuum pump 109 and, e.g., an argon gas is introduced into the chamber 101 as a gas for generating a plasma whose pressure falls within a predetermined range of pressure.
The microwave generated by the high frequency power source 105 is propagated through the waveguide 106 to the antenna unit 103. As shown in FIG. 23, the microwave that has reached the antenna unit 103 propagates through the wave delay plate 103b as indicated by arrows and is radiated through the slot plate 103c into the chamber 101 to generate an electromagnetic field.
Due to the electromagnetic field generated in the chamber 101, the argon gas is dissociated and a plasma generation region 122 is formed between the substrate 111 and the top plate 104 so that a predetermined plasma processing is performed on the substrate 111.
In the plasma generation region 122 formed in the chamber 101, to maintain the plasma generation region 122 electrically neutral, electrons and ions (charged particles) in the plasma generation region 122 are made to vibrate at a certain plasma frequency. The plasma frequency increases as the density of the charged particles becomes higher and the mass of the charged particles becomes lighter.
Accordingly, in the plasma generation region 122, the plasma frequency of electrons having a mass substantially smaller than that of ions is considerably higher than the plasma frequency of the ions and falls in a microwave range. At this time, in case the frequency of the microwave generated by the high frequency power source 105 is greater than the plasma frequency, the microwave can propagate through the plasma generation region 122 so that it is possible to supply the microwave to the plasma generation region 122.
Meanwhile, the plasma frequency of the electrons increases as the density of the electrons becomes higher; and if the plasma frequency becomes higher than that of the microwave generated by the high frequency power source 105, i.e., if a cutoff frequency in the plasma generation region 122 becomes higher than the frequency of the microwave, there occurs a phenomenon that the electric field of the microwave is blocked at the periphery of the plasma generation region 122. That is, the microwave is reflected by the plasma generation region 122. As the electron density becomes higher, such phenomenon prominently occurs.
As illustrated in FIG. 23, a part of the microwave reflected by the plasma generation region 122 propagates through the top plate 104. As a result, a strong electric field is generated at a location 130 adjacent to an outer peripheral portion of the top plate 104 wherein the top plate 104 contacts with the chamber 101, thereby resulting in an abnormal discharge and a production of foreign materials.